Surface Chemistry of Alanine on Ni{111} Supporting Information

Transcription

1 S1 Surface Chemistry of Alanine on Ni{111} Richard E. J. Nicklin 1, Alix Cornish 1, Andrey Shavorskiy 2, Silvia Baldanza 1, Karina Schulte 3, Zhi Liu 2, Roger A. Bennett 1, Georg Held 1,4 1 Department of Chemistry, University of Reading, Reading RG6 6AD, UK 2 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA 3 MAX-lab, Lund University, Lund, Sweden 4 Diamond Light Source Ltd, Oxfordshire OX11 0DE, UK Supporting Information Corresponding address: University of Reading, Department of Chemistry Whiteknights, Reading RG6 6AD UK g.held@reading.ac.uk

2 Fitting Parameters S2 C1s N1s O1s -COOH -COO -CN -NH2 -NH + 3 -COO -CNH + 3 -CNH2/-CH3 -CO -C=C -C/-CN 250 K / Multilayers Area BE FWHM K / 0.26 ML Area BE FWHM K / 0.25 ML Area BE FWHM K / 0.18 ML Area BE FWHM K / 0.11 ML Area BE FWHM K / 0.03 ML Area BE FWHM The peak labelled -CNH + 3 also contains contributions from an amine fragment. The layer labelled 0.26 ML already shows signs of multilayer growth. The coverage is therefore not accurate. Table S1: Key fit parameters of the XP spectra in Figure 1 a.,b.,c. (adsorption of alanine at 250 K) of the main text: peak area (arb. units), binding energy (BE / ev), full width of half maximum (FWHM / ev).

3 S3 C1s N1s O1s -COOH -COO -CN -NH2 -NH + 3 -COO -CNH + 3 -CNH2/-CH3 -CO -C=C -C/-CN 300 K / 0.26 ML Area BE FWHM K / 0.25 ML Area BE FWHM K / 0.20 ML Area BE FWHM K / 0.19 ML Area BE FWHM K / 0.16 ML Area BE FWHM K / 0.15 ML Area BE FWHM K / 0.07 ML Area BE FWHM The peak labelled -CNH + 3 also contains contributions from an amine fragment. Table S2: Key fit parameters of the XP spectra in Figure 1 d.,e.,f. (adsorption of alanine at 300 K) of the main text: peak area (arb. units), binding energy (BE / ev), full width of half maximum (FWHM / ev).

4 S4 NEXAFS data analysis NEXAFS spectra were fitted with a series of Gaussian and Lorentzian functions (one for each discernable peak), with Lorentzian functions being substituted for sharp features of width 1 ev, superimposed on an arctan function to capture the step: f(e) = B off + S h arctan( E S i=n P ) + S W i=1 G i (E) + j=m j=1 L j (E) (1) where f is the NEXAFS fitting function, B off is the intensity offset, S h the step height, E the photon energy, S p the mid-point of the step, G i the Gaussian function for peak i (of which there are N), L j the Lorentzian of peak j (of which there are M). Adapted from [1]. No corrections were made for energy offsets or intensity decays. Exact photon energy calibration is not necessary as only the intensity of the π-resonances is taken into account for the data analysis; intensity variations due to ring current decay are not expected to be a major effect, as the decay was 5% over the course of a NEXAFS experiment. The abundance of structure with apparent angular dependence results in a step position that is particularly hard to define (this may be due to the presence of several carbon species on the surface, some of which will be tightly bound). Two angles of incidence were employed when examining angular dependence. This is mathematically sufficient to determine the angle between the plane of the COO group and a surface with three-fold symmetry (α) via a simultaneous solution of the equation [1]: [ I = I v = A P (sin ϑ) 2 (1 3 2 sin2 α) + 1 ] 2 sin2 α (2) δ is the angle of incidence of the photon beam with respect to the surface normal, and P the polarisation of the beam (P 1 at these photon energies). Such a method is prone to errors, however, and should be taken as an indication rather than a definite measurement. The I 0 values recorded as a function of photon energy in both the oxygen K-edge and

5 S5 particularly carbon K-edge regions ( ev and ev respectively) exhibit pronounced structures near the position of the π resonances due to mirror contamination. The raw spectra were divided by the I 0 spectra in order to compensate for these variations. This did, however, not eliminate all artificial structures, particularly from the carbon spectra. The structures in the carbon spectra must therefore be considered with caution. It is likely that not only are diverse chemical species contributing structure (as decomposition products will contribute to the NEXAFS spectrum as well as the amino acid) but also instrumental effects. For all spectra, most effort has been devoted to obtaining good fits for the low photon energy region near to the π resonances, as this is the strongest, sharpest feature and will convey most accurately information regarding the orientation of the adsorbed L-alanine. For fitting purposes, the absorption step edge was prevented from taking a value 3eV higher than the maximum binding energy found from XPS experiments.

6 S6 Carbon and Nitrogen K-edge NEXAFS Coverage 0.21 ML Sample Temperature 250 K 10.0 Intensity (arb. units) Normal Incidence 65 º Photon Energy / ev Figure S1: Carbon K-edge NEXAFS spectra of 0.21 ML L-alanine on Ni{111} adsorbed at 250 K. Recorded the same conditions as those for the O and N K-edge spectra presented in the main body of the paper. The angles of incidence are shown in the Figure. These spectra show structure below the step edge, which is much likely arising from contamination of the beamline optical components. As a result, fitting of these NEXAFS is not presented.

7 S7 Figure S2: Nitrogen K-edge NEXAFS recorded from annealed layers of L-alanine on Ni{111}. The initial coverage was 0.25 ML. The angles of incidence and annealing temperatures are shown in the Figure.

8 S8 Temperature Programmed Desorption and XPS Figure S3: TPD spectra recorded from L-alanine on Ni{111} between 240 K K at a heating rate of 1 K s 1. L-alanine was dosed at a sample temperature of 250 K.

9 S a.) 0.18 ML b.) 0.15 ML c.) 0.21 ML Temperature / K 500 Temperature / K 500 Temperature / K (a.) Binding Energy / ev (b.) Binding Energy / ev (c.) Binding Energy / ev Figure S4: TP-XPS spectra recorded from L-alanine on Ni{111}, dosed at sample temperatures 200 K (b.) and 250 K (a., c.) with a heating rate of 0.2 K s 1 References [1] J. Stöhr, NEXAFS spectroscopy, Springer series in Surface Sciences, Springer, Berlin (1996).